One of the steps commonly required for the synthesis of a heterogeneous catalyst involves the deposition of a component generically known as a “promoter,” which might be an anionic, cationic, or molecular species, onto a surface of a catalytic material. The deposition process is usually intended to disperse the promoter broadly and uniformly.
The promoter is sometimes deposited out of the gas phase, as for example when HCl carried by an inert gas is reacted with a metal oxide, or when a metal is introduced by vaporizing its volatile carbonyl compounds into a carrier gas and decomposing the metal carbonyl onto the surface of a catalyst precursor.
In some situations a promoter that is ionic is deposited onto a catalyst precursor from solution in a polar solvent by ion exchange. This process requires that the catalyst precursor carry ionic groups on its surface so that a species that is usually a surface cation can be exchanged by a different cation out of a concentrated solution. This process is employed, for example, when a zeolite bearing oxide anions bound to the zeolite framework and neutralized by sodium countercations is converted to the acid form with proton countercations by exchange with aqueous ammonium cations followed by calcination to drive off ammonia.
Adsorption is a related process wherein a catalyst precursor extracts the promoter out of solution. Van der Waals forces can attach the promoter to the catalyst precursor (no exchangeable ions are involved).
Another common method for introducing a promoter onto a catalyst precursor involves dissolving a promoter in a solvent that may be water or some other fluid, and then depositing the promoter from the solution by evaporating the solvent. If the volume of solvent involved is about equal to the catalyst precursor pore volume, then the procedure is called “incipient wetness impregnation.” Alternately, the catalyst precursor can be stirred with an excess volume of solution and the slurry evaporated to dryness so that the promoter originally in solution is deposited onto the catalyst surface. If enough solvent is used to cause the catalyst precursor to be conspicuously wetted, then the procedure is known as “solvent impregnation.”
Finally, a promoter is sometimes added by dry-mixing the promoter and catalyst precursor solids and allowing the promoter to migrate to the catalyst surface as it is melted, volatilized, or otherwise rendered mobile inside the catalytic reactor in the presence of heat, reactants, and products.
Each of these procedures has limitations. Promoter deposition from the gas phase requires that the promoter have a volatile form. The ion-exchange method requires that both the catalyst precursor and the promoter be ionic, and further that the ion-exchange sites be the desired destination for the promoter. Adsorption forces may not be sufficiently strong. The deposition of a promoter from excess solvent by impregnation can lead to surface chemistry that may not be beneficial.
In the dry-mix approach, there is little or no control over the delivery of the promoter to the catalyst surface. In situ dispersal of a promoter that has been dry-mixed requires an induction period during which the catalyst is not effectively promoted.
In view of these limitations, improved methods of introducing promoters to catalysts are needed. Specifically, there is a need for practical methods to disperse promoters onto the surface of catalysts or catalyst precursors, wherein the dispersion leads to a substantially uniform promoter composition and efficient use of the promoter.
While solvent impregnation can give good promoter dispersion, there is a desire to minimize the use of solvents for several reasons: (i) potential damage to the catalyst by solvent contact is minimized, (ii) energy needs for later solvent removal are reduced, and (iii) waste solvent streams are minimized. It would be particularly advantageous to combine the practical benefits of dry-mixing a promoter and catalyst precursor, with the enhanced dispersion that can be realized by solvent-based methods.